Young Folks' Library, Volume XI (of 20) eBook

SOME RECORDS OF THE ROCKS

(FROM A FIRST BOOK IN GEOLOGY.)

BY N.S. SHALER, S.D.[1]

[Footnote 1: Copyright, 1884, by N.S. Shaler.]

[Illustration]

The geologist cannot find his way back in the record
of the great stone book, to the far-off day when life
began. The various changes that come over rocks
from the action of heat, of water, and of pressure,
have slowly modified these ancient beds, so that they
no longer preserve the frames of the animals that
were buried in them.

These old rocks, which are so changed that we cannot
any longer make sure that any animals lived in them,
are called the “archaean,” which is Greek
for ancient. They were probably mud and sand and
limestone when first made, but they have been changed
to mica schists, gneiss, granite, marble, and other
crystalline rocks. When any rock becomes crystalline,
the fossils dissolve and disappear, as coins lose their
stamp and form when they are melted in the jeweller’s
gold-pot.

These ancient rocks that lie deepest in the earth
are very thick, and must have taken a great time in
building; great continents must have been worn down
by rain and waves in order to supply the waste out
of which they were made. It is tolerably certain
that they took as much time during their making as
has been required for all the other times since they
were formed. During the vast ages of this archaean
the life of our earth began to be. We first find
many certain evidences of life in the rocks which
lie on top of the archaean rock, and are known as
the Cambriani and Silurian periods. There we have
creatures akin to our corals and crabs and worms,
and others that are the distant kindred of the cuttle-fishes
and of our lamp-shells. There were no backboned
animals, that is to say, no land mammals, reptiles,
or fishes at this stage of the earth’s history.
It is not likely that there was any land life except
of plants and those forms like the lowest ferns, and
probably mosses. Nor is it likely that there were
any large continents as at the present time, but rather
a host of islands lying where the great lands now
are, the budding tops of the continents just appearing
above the sea.

Although the life of this time was far simpler than
at the present day, it had about as great variety
as we would find on our present sea-floors. There
were as many different species living at the same
time on a given surface.

The Cambrian and Silurian time—­the time
before the coming of the fishes—­must have
endured for many million years without any great change
in the world. Hosts of species lived and died;
half a dozen times or more the life of the earth was
greatly changed. New species came much like those
that had gone before, and only a little gain here
and there was perceptible at any time. Still,
at the end of the Silurian, the life of the world
had climbed some steps higher in structure and in
intelligence.